In recent years, lithium-ion secondary batteries have established their status as high capacity and lightweight power sources that are indispensable to portable electronic terminals, electric vehicles, etc. On the other hand, sodium-ion secondary batteries using sodium as substitute for lithium are also studied because there is concern of the global escalating cost of raw materials for lithium used in lithium-ion secondary batteries.
To achieve high energy density, alkali-ion secondary batteries are required to have higher voltage or higher capacity. However, the reference redox potential of sodium ions is higher than that of lithium ions by 0.3 V. When alkali ions of the positive electrode active material are changed from lithium to sodium, therefore, the operating potential decreases. Thus, in order to achieve high energy density comparable with that of lithium ion secondary batteries, needs for high voltage or high capacity are relatively high in sodium-ion secondary batteries. For example, Non-Patent Document 1 discloses a positive electrode active material composed of Na2(Fe1-yMny)P2O7 (0≤y≤1).